A closed list that opens downward when asked. It hides its options until they are wanted, then shows them all at once.
A modern compound of drop (Old English dropa, a fall) and down (Old English dūn, downward); a list that falls open rather than sitting exposed.
Its own name says the fall twice over, once as a drop and once as down, as if the direction needed stating for both the ear and the eye. What it hides stays hidden until asked for, then arrives all at once rather than one option at a time.